Many years ago I had a cousin parish by welding an "empty" tank that had
the vapors sucked or blown out in some fashion.  I remember the
investigator suggested the use of Dry Ice (frozen Carbon Dioxide)placed
inside the tank would eliminate the possibility of combustion or
explosion.  We were told then that the use of Dry Ice was the typical
practice when reworking fuel tanks.  For what it's worth.

Joe Trepicone